The role of CXCL12 in the organ-specific process of artery formation

The CXCR4/CXCR7/SDF-1 pathway contributes to the pathogenesis of Shiga toxin-associated hemolytic uremic syndrome in humans and mice

Hemolytic uremic syndrome (HUS) is a potentially life-threatening condition. It often occurs after gastrointestinal infection with E. coli O157:H7, which produces Shiga toxins (Stx) that cause hemolytic anemia, thrombocytopenia, and renal injury. Stx-mediated changes in endothelial phenotype have been linked to the pathogenesis of HUS. Here we report our studies investigating Stx-induced changes in gene expression and their contribution to the pathogenesis of HUS. Stx function by inactivating host ribosomes but can also alter gene expression at concentrations that minimally affect global protein synthesis. Gene expression profiling of human microvascular endothelium treated with Stx implicated a role for activation of CXCR4 and CXCR7 by their shared cognate chemokine ligand (stromal cell-derived factor-1 [SDF-1]) in Stx-mediated pathophysiology. The changes in gene expression required a catalytically active Stx A subunit and were mediated by enhanced transcription and mRNA stability. Stx also enhanced the association of CXCR4, CXCR7, and SDF1 mRNAs with ribosomes. In a mouse model of Stx-mediated pathology, we noted changes in plasma and tissue content of CXCR4, CXCR7, and SDF-1 after Stx exposure. Furthermore, inhibition of the CXCR4/SDF-1 interaction decreased endothelial activation and organ injury and improved animal survival. Finally, in children infected with E. coli O157:H7, plasma SDF-1 levels were elevated in individuals who progressed to HUS. Collectively, these data implicate the CXCR4/CXCR7/SDF-1 pathway in Stx-mediated pathogenesis and suggest novel therapeutic strategies for prevention and/or treatment of complications associated with E. coli O157:H7 infection.

The CXCR4/CXCR7/SDF-1 pathway contributes to the pathogenesis of Shiga toxin-associated hemolytic uremic syndrome in humans and mice

Hemolytic uremic syndrome (HUS) is a potentially life-threatening condition. It often occurs after gastrointestinal infection with E. coli O157:H7, which produces Shiga toxins (Stx) that cause hemolytic anemia, thrombocytopenia, and renal injury. Stx-mediated changes in endothelial phenotype have been linked to the pathogenesis of HUS. Here we report our studies investigating Stx-induced changes in gene expression and their contribution to the pathogenesis of HUS. Stx function by inactivating host ribosomes but can also alter gene expression at concentrations that minimally affect global protein synthesis. Gene expression profiling of human microvascular endothelium treated with Stx implicated a role for activation of CXCR4 and CXCR7 by their shared cognate chemokine ligand (stromal cell-derived factor-1 [SDF-1]) in Stx-mediated pathophysiology. The changes in gene expression required a catalytically active Stx A subunit and were mediated by enhanced transcription and mRNA stability. Stx also enhanced the association of CXCR4, CXCR7, and SDF1 mRNAs with ribosomes. In a mouse model of Stx-mediated pathology, we noted changes in plasma and tissue content of CXCR4, CXCR7, and SDF-1 after Stx exposure. Furthermore, inhibition of the CXCR4/SDF-1 interaction decreased endothelial activation and organ injury and improved animal survival. Finally, in children infected with E. coli O157:H7, plasma SDF-1 levels were elevated in individuals who progressed to HUS. Collectively, these data implicate the CXCR4/CXCR7/SDF-1 pathway in Stx-mediated pathogenesis and suggest novel therapeutic strategies for prevention and/or treatment of complications associated with E. coli O157:H7 infection.

Progenitor cell mobilization and recruitment: SDF-1, CXCR4, α4-integrin, and c-kit

Progenitor cell retention and release are largely governed by the binding of stromal-cell-derived factor 1 (SDF-1) to CXC chemokine receptor 4 (CXCR4) and by α4-integrin signaling. Both of these pathways are dependent on c-kit activity: the mobilization of progenitor cells in response to either CXCR4 antagonism or α4-integrin blockade is impaired by the loss of c-kit kinase activity; and c-kit-kinase inactivation blocks the retention of CXCR4-positive progenitor cells in the bone marrow. SDF-1/CXCR4 and α4-integrin signaling are also crucial for the retention of progenitor cells in the ischemic region, which may explain, at least in part, why clinical trials of progenitor cell therapy have failed to display the efficacy observed in preclinical investigations. The lack of effectiveness is often attributed to poor retention of the transplanted cells and, to date, most of the trial protocols have mobilized cells with injections of granulocyte colony-stimulating factor (G-CSF), which activates extracellular proteases that irreversibly cleave cell-surface adhesion molecules, including α4-integrin and CXCR4. Thus, the retention of G-CSF-mobilized cells in the ischemic region may be impaired, and the mobilization of agents that reversibly disrupt SDF-1/CXCR4 binding, such as AMD3100, may improve patient response. Efforts to supplement SDF-1 levels in the ischemic region may also improve progenitor cell recruitment and the effectiveness of stem cell therapy.

Mast cell accumulation in glioblastoma with a potential role for stem cell factor and chemokine CXCL12

Glioblastoma multiforme (GBM) is the most common and malignant form of glioma with high mortality and no cure. Many human cancers maintain a complex inflammatory program triggering rapid recruitment of inflammatory cells, including mast cells (MCs), to the tumor site. However, the potential contribution of MCs in glioma has not been addressed previously. Here we report for the first time that MCs infiltrate KRas+Akt-induced gliomas, using the RCAS/TV-a system, where KRas and Akt are transduced by RCAS into the brains of neonatal Gtv-a- or Ntv-a transgenic mice lacking Ink4a or Arf. The most abundant MC infiltration was observed in high-grade gliomas of Arf−/− mice. MC accumulation could be localized to the vicinity of glioma-associated vessels but also within the tumor mass. Importantly, proliferating MCs were detected, suggesting that the MC accumulation was caused by local expansion of the MC population. In line with these findings, strong expression of stem cell factor (SCF), i.e. the main MC growth factor, was detected, in particular around tumor blood vessels. Further, glioma cells expressed the MC chemotaxin CXCL12 and MCs expressed the corresponding receptor, i.e. CXCR4, suggesting that MCs could be attracted to the tumor through the CXCL12/CXCR4 axis. Supporting a role for MCs in glioma, strong MC infiltration was detected in human glioma, where GBMs contained significantly higher MC numbers than grade II tumors did. Moreover, human GBMs were positive for CXCL12 and the infiltrating MCs were positive for CXCR4. In conclusion, we provide the first evidence for a role for MCs in glioma.

The role of chemokines and their receptors in angiogenesis

Chemokines are a vertebrate-specific group of small molecules that regulate cell migration and behaviour in diverse contexts. So far, around 50 chemokines have been identified in humans, which bind to 18 different chemokine receptors. These are members of the seven-transmembrane receptor family. Initially, chemokines were identified as modulators of the immune response. Subsequently, they were also shown to regulate cell migration during embryonic development. Here, we discuss the influence of chemokines and their receptors on angiogenesis, or the formation of new blood vessels. We highlight recent advances in our understanding of how chemokine signalling might directly influence endothelial cell migration. We furthermore examine the contributions of chemokine signalling in immune cells during this process. Finally, we explore possible implications for disease settings, such as chronic inflammation and tumour progression.

Arterial-venous network formation during brain vascularization involves hemodynamic regulation of chemokine signaling

During angiogenic sprouting, newly forming blood vessels need to connect to the existing vasculature in order to establish a functional circulatory loop. Previous studies have implicated genetic pathways, such as VEGF and Notch signaling, in controlling angiogenesis. We show here that both pathways similarly act during vascularization of the zebrafish central nervous system. In addition, we find that chemokine signaling specifically controls arterial-venous network formation in the brain. Zebrafish mutants for the chemokine receptor cxcr4a or its ligand cxcl12b establish a decreased number of arterial-venous connections, leading to the formation of an unperfused and interconnected blood vessel network. We further find that expression of cxcr4a in newly forming brain capillaries is negatively regulated by blood flow. Accordingly, unperfused vessels continue to express cxcr4a, whereas connection of these vessels to the arterial circulation leads to rapid downregulation of cxcr4a expression and loss of angiogenic characteristics in endothelial cells, such as filopodia formation. Together, our findings indicate that hemodynamics, in addition to genetic pathways, influence vascular morphogenesis by regulating the expression of a proangiogenic factor that is necessary for the correct pathfinding of sprouting brain capillaries.

Roles of cyclic adenosine monophosphate signaling in endothelial cell differentiation and arterial-venous specification during vascular development

Cyclic adenosine monophosphate (cAMP) is an important second messenger mediating physiological functions, including metabolism, gene expression, cell growth and differentiation. Recently, we demonstrated novel roles of cAMP pathway in endothelial cell (EC) differentiation and arterial-venous specification using an embryonic stem cell differentiation system. These studies offered a concept that vascular formation is accomplished by a 2-layered mechanism: (1) a basal mechanism for common EC differentiation, whereby vascular endothelial growth factor (VEGF) signaling plays a central role in the basal mechanism, and (2) a vascular diversification mechanism working on the basis of common EC differentiation. Vascular diversification, such as artery and vein formation, can be only achieved by enacting specific machineries in the presence of the basal EC machinery. cAMP/protein kinase A signaling contributes to common EC differentiation through upregulation of the VEGF-A receptors, Flk1 and neuropilin1. On the other hand, cAMP can activate phosphatidylinositol-3 kinase, which induces an arterial fate in vascular progenitors via dual activation of Notch and β-catenin signaling as an arterial-specific machinery. cAMP signaling thus plays a pivotal role in both the basal and diversification machinery during vascular development.

Chemokines in angiogenesis

Chemokines are a family of small heparin-binding proteins, mostly known for their role in inflammation and immune surveillance, which have emerged as important regulators of angiogenesis. Chemokines influence angiogenesis either through recruitment of pro-angiogenic immune cells and endothelial progenitors to the neo-vascular niche or via direct regulation of endothelial function downstream of activation of G-protein coupled chemokine receptors. The dual function of chemokines in regulating immune response and angiogenesis confers a central role in modulating the tissue microenvironment. Therefore, chemokines may constitute attractive targets for therapeutic intervention in several pathological disorders. This review will summarize the current understanding of the role of chemokines in angiogenesis, and give an overview of angiostatic and angiogenic chemokines and their crosstalk with other angiogenic factors.

Essential regulation of CNS angiogenesis by the orphan G protein-coupled receptor GPR124

The orphan G protein-coupled receptor (GPCR) GPR124/tumor endothelial marker 5 is highly expressed in central nervous system (CNS) endothelium. Here, we show that complete null or endothelial-specific GPR124 deletion resulted in embryonic lethality from CNS-specific angiogenesis arrest in forebrain and neural tube. Conversely, GPR124 overexpression throughout all adult vascular beds produced CNS-specific hyperproliferative vascular malformations. In vivo, GPR124 functioned cell-autonomously in endothelium to regulate sprouting, migration, and developmental expression of the blood-brain barrier marker Glut1, whereas in vitro, GPR124 mediated Cdc42-dependent directional migration to forebrain-derived, vascular endothelial growth factor-independent cues. Our results demonstrate CNS-specific angiogenesis regulation by an endothelial receptor and illuminate functions of the poorly understood adhesion GPCR subfamily. Further, the functional tropism of GPR124 marks this receptor as a therapeutic target for CNS-related vascular pathologies.

DOCK180 is a Rac activator that regulates cardiovascular development by acting downstream of CXCR4

RATIONALE

During embryogenesis, the CXC chemokine ligand (CXCL)12 acts on endothelial cells to control cardiac development and angiogenesis. Although biological functions of CXCL12 are exerted in part through activation of the small GTPase Rac, the pathway leading from its receptor CXC chemokine receptor (CXCR)4 to Rac activation remains to be determined.

OBJECTIVE

DOCK180 (dedicator of cytokinesis), an atypical Rac activator, has been implicated in various cellular functions. Here, we examined the role of DOCK180 in cardiovascular development.

METHODS AND RESULTS

DOCK180 associates with ELMO (engulfment and cell motility) through the N-terminal region containing a Src homology 3 domain. We found that targeted deletion of the Src homology 3 domain of DOCK180 in mice leads to embryonic lethality with marked reduction of DOCK180 expression at the protein level. These mutant mice, as well as DOCK180-deficient mice, exhibited multiple cardiovascular abnormalities resembling those seen in CXCR4-deficient mice. In DOCK180 knocked down endothelial cells, CXCL12-induced Rac activation was impaired, resulting in a marked reduction of cell motility.

CONCLUSIONS

These results suggest that DOCK180 links CXCR4 signaling to Rac activation to control endothelial cell migration during cardiovascular development.

Convergence of Notch and beta-catenin signaling induces arterial fate in vascular progenitors

The Notch intracellular domain and β-catenin team up with RBP-J to regulate gene transcription and promote the development of arterial endothelial cells.

Molecular mechanisms controlling arterial–venous specification have not been fully elucidated. Previously, we established an embryonic stem cell differentiation system and demonstrated that activation of cAMP signaling together with VEGF induces arterial endothelial cells (ECs) from Flk1⁺ vascular progenitor cells. Here, we show novel arterial specification machinery regulated by Notch and β-catenin signaling. Notch and GSK3β-mediated β-catenin signaling were activated downstream of cAMP through phosphatidylinositol-3 kinase. Forced activation of Notch and β-catenin with VEGF completely reconstituted cAMP-elicited arterial EC induction, and synergistically enhanced target gene promoter activity in vitro and arterial gene expression during in vivo angiogenesis. A protein complex with RBP-J, the intracellular domain of Notch, and β-catenin was formed on RBP-J binding sites of arterial genes in arterial, but not venous ECs. This molecular machinery for arterial specification leads to an integrated and more comprehensive understanding of vascular signaling.

Microarray analysis of retinal endothelial tip cells identifies CXCR4 as a mediator of tip cell morphology and branching

The development of the vertebrate vascular system is mediated by both genetic patterning of vessels and by angiogenic sprouting in response to hypoxia. Both of these processes depend on the detection of environmental guidance cues by endothelial cells. A specialized subtype of endothelial cell known as the tip cell is thought to be involved in the detection and response to these cues, but the molecular signaling pathways used by tip cells to mediate tissue vascularization remain largely uncharacterized. To identify genes critical to tip cell function, we have developed a method to isolate them using laser capture microdissection, permitting comparison of RNA extracted from endothelial tip cells with that of endothelial stalk cells using microarray analysis. Genes enriched in tip cells include ESM-1, angiopoietin-2, and SLP-76. CXCR4, a receptor for the chemokine stromal-cell derived factor-1, was also identified as a tip cell-enriched gene, and we provide evidence for a novel role for this receptor in mediating tip cell morphology and vascular patterning in the neonatal retina.

Hepatic stellate cells promote liver metastasis of colon cancer cells by the action of SDF-1/CXCR4 axis

BACKGROUND

It has been determined that the chemokine receptor CXCR4 and its ligand stromal cell-derived factor-1 (SDF-1) regulate several key processes in a wide variety of cancers. However, the function and mechanism of the SDF-1/CXCR4 system in the metastasis of colorectal cancer remain controversial.

METHODS

Immunohistochemistry was performed to examine quantitatively the expression of CXCR4 in 40 human samples of colorectal cancer and liver metastasis. The functions of SDF-1 on HCT116 colon cancer cells were investigated in vitro. We subcutaneously inoculated HCT116 cells with hepatic stellate cells (HSCs) expressing SDF-1. The CXCR4 inhibitor AMD3100 was tested in vitro and in vivo.

RESULTS

By quantitatively counting the number of cells, it was shown that there are more CXCR4-positive cells at the metastatic site in the liver compared with the primary sites. We demonstrated the effect of SDF-1 on the invasion and antiapoptosis of HCT116 cells in vitro. In mouse experiment of liver metastasis, intraperitoneal administration of AMD3100 blocked the metastatic potential of HCT116 cells. Furthermore, we found that alpha-smooth muscle actin (alpha-SMA)-positive myofibroblasts derived from HSCs, surrounding the liver metastasis foci, secreted SDF-1. The subcutaneous inoculation of HCT116 cells with HSCs promoted the tumor initiation in nude mice, indicating the importance of the direct interaction between these cells in vivo.

CONCLUSION

These results suggest that HSCs play important role in liver metastasis of colon cancer cells by the action of SDF-1/CXCR4 axis and provide preclinical evidence that blockade of the axis is a target for antimetastasis therapy.

The CXCL12 (SDF-1)/CXCR4 axis is essential for the development of renal vasculature

CXC chemokine ligand 12 (CXCL12; stromal cell-derived factor 1) is a unique homeostatic chemokine that signals through its cognate receptor, CXCR4. CXCL12/CXCR4 signaling is essential for the formation of blood vessels in the gastrointestinal tract during development, but its contribution to renal development remains unclear. Here, we found that CXCL12-secreting stromal cells surround CXCR4-positive epithelial components of early nephrons and blood vessels in the embryonic kidney. In glomeruli, we observed CXCL12-secreting podocytes in close proximity to CXCR4-positive endothelial cells. Both CXCL12- and CXCR4-deficient kidneys exhibited identical phenotypes; there were no apparent abnormalities in early nephrogenesis or in differentiation of podocytes and tubules, but there was defective formation of blood vessels, including ballooning of the developing glomerular tuft and disorganized patterning of the renal vasculature. To clarify the relative importance of different cellular defects resulting from ablation of CXCL12 and CXCR4, we established endothelial cell-specific CXCR4-deficient mice, which recapitulated the renal phenotypes of conventional CXCR4-deficient mice. We conclude that CXCL12 secreted from stromal cells or podocytes acts on endothelial cells to regulate vascular development in the kidney. These findings suggest new potential therapeutic targets for remodeling the injured kidney.

The angiogenic response is dictated by beta3 integrin on bone marrow-derived cells

Angiogenesis is dependent on the coordinated action of numerous cell types. A key adhesion molecule expressed by these cells is the α_vβ₃ integrin. Here, we show that although this receptor is present on most vascular and blood cells, the key regulatory function in tumor and wound angiogenesis is performed by β₃ integrin on bone marrow–derived cells (BMDCs) recruited to sites of neovascularization. Using knockin mice expressing functionally stunted β₃ integrin, we show that bone marrow transplantation rescues impaired angiogenesis in these mice by normalizing BMDC recruitment. We demonstrate that α_vβ₃ integrin enhances BMDC recruitment and retention at angiogenic sites by mediating cellular adhesion and transmigration of BMDCs through the endothelial monolayer but not their release from the bone niche. Thus, β₃ integrin has the potential to control processes such as tumor growth and wound healing by regulating BMDC recruitment to sites undergoing pathological and adaptive angiogenesis.

The roles of chemokine CXCL12 in embryonic and brain tumor angiogenesis

The formation of blood vessels in embryos and tumors are different processes but under the control of common molecular mechanisms. Chemokine CXCL12 involved in both embryonic and tumor angiogenesis. In this review, we summarize recent advances in understanding the roles of CXCL12 in brain tumor angiogenesis/vasculogenesis. CXCL12 and its cognate receptors are abnormally induced in brain tumors, in particular in tumor cells and endothelium. Pathologically enhanced CXCL12 signaling may promote the formation of new vessels through recruiting circulating endothelial progenitor cells or directly enhancing the migration/growth of endothelial cells. Therefore, CXCL12 signaling represents an important mechanism that regulates brain tumor angiogenesis/vasculogenesis and may provide potential targets for anti-angiogenic therapy in malignant gliomas.

Regulation of axonal elongation and pathfinding from the entorhinal cortex to the dentate gyrus in the hippocampus by the chemokine stromal cell-derived factor 1 alpha

During the early developmental stage, a neural circuit is established between the entorhinal cortex (EC) and the hippocampal dentate gyrus (DG) via the perforant pathway. However, the manner in which the perforant fibers are navigated has mostly remained a mystery. Here, we analyzed the functional role of a chemokine, namely, stromal cell-derived factor 1alpha (SDF-1alpha), in the navigation of the perforant fibers. SDF-1alpha was observed to promote neurite growth, which is dependent on mDia1, in cultured entorhinal cortical neurons obtained from rats at postnatal day 0. We then used entorhino-hippocampal cocultures comprising green fluorescence-labeled EC and DG slices to assess the projection of the perforant fibers from the EC. Although the specific laminar termination of the entorhinal axons was observed with this system, the number of appropriately terminating entorhinal axons decreased significantly when the SDF-1alpha signaling pathway was blocked by a neutralizing antibody against SDF-1alpha or by the specific SDF-1alpha receptor antagonist AMD3100 (1,1'-[1,4-phenylenebis(methylene)]bis-1,4,8,11-tetra-azacyclotetradecane octahydrochloride). Furthermore, inhibition of the SDF-1alpha signaling pathway resulted in a decrease in the immunoreactivity for PSD-95 (postsynaptic density protein-95) in the DG, possibly because of a reduction in the number of projecting perforant fibers. These results demonstrate that SDF-1alpha plays a critical role in promoting the growth of perforant fibers from the EC to the DG.

Stromal cell-derived factor-1 stimulates vasculogenesis and enhances Ewing's sarcoma tumor growth in the absence of vascular endothelial growth factor

Stromal cell-derived Factor-1alpha (SDF-1alpha) stimulates the migration of bone marrow (BM) cells, similar to vascular endothelial growth factor (VEGF). We previously demonstrated that inhibition of VEGF(165) by small interfering RNA inhibited Ewing's sarcoma tumor growth, tumor vessel formation and recruitment of BM cells to the tumor. To determine the importance of BM cells in tumor vessel development, we investigated the effects of SDF-1alpha on VEGF-inhibited TC/siVEGF(7-1) Ewing's tumor neovasculature formation and growth. The effect of SDF-1alpha on CD34(+) progenitor cell chemotaxis was determined in vivo. Using a BM transplantation model with GFP(+) transgenic mice as BM donors and nude mice as recipients, we evaluated the effect of SDF-1alpha on the recruitment of BM-derived cells to VEGF(165)-inhibited TC/siVEGF(7-1) tumors, as well as its effect on neovasculature development, vessel morphology and tumor growth. SDF-1alpha stimulated the migration of CD34(+) progenitor cells to Matrigel plugs in vivo and promoted the retainment of BM-derived pericytes in close association with perfused, functional tumor vessels. Intratumor inoculation of Ad-SDF-1alpha into TC/siVEGF(7-1) tumors resulted in increased SDF-1 and PDGF-BB expression, augmented tumor growth, an increase in the number of large, lumen-bearing vascular structures, and enhanced vessel pericyte coverage, with no change in VEGF(165). SDF-1alpha stimulates BM cell chemotaxis and the association of these cells with functional tumor vessels. Furthermore, SDF-1alpha enhances tumor neovascularization and growth with no alteration in VEGF(165). Our work suggests that SDF-1-mediated vasculogenesis may represent an alternate pathway that could potentially be utilized by tumors to sustain growth and neovasculature expansion after anti-VEGF therapy.

Forkhead transcription factors regulate expression of the chemokine receptor CXCR4 in endothelial cells and CXCL12-induced cell migration

Foxc1 and Foxc2 transcription factors are required for vascular development. However, the molecular mechanisms by which Foxc1 and Foxc2 control angiogenesis, the growth of new blood vessels from pre-existing vessels and capillaries, remain unknown. CXC chemokine ligand 12 (CXCL12) and its receptor, CXCR4, are critical for the process of angiogenesis, including the migration and tube formation of endothelial cells. Here we show that Foxc1 and Foxc2 directly induce CXCR4 expression by activating its promoter in endothelial cells. Furthermore, Foxc1-deficient endothelial cells show a significant reduction in CXCR4 expression as well as CXCL12-stimulated migration. Taken together, these results provide novel evidence that Foxc transcription factors are important regulators of the chemotactic motility of endothelial cells through the induction of CXCR4 expression.

Expression of SDF-1/CXCR4 in injured human kidneys

The chemokine SDF-1alpha is involved in migration, survival, and development of multiple cells, most notably of hematopoietic stem cells (HSC) expressing its ligand CXCR4. Recently, we have shown engraftment of human HSC in the ischemically injured murine kidney, presumably mediated by SDF-1alpha. To further investigate a possible role of SDF-1alpha in the recruitment of CXCR4(+) cells in human renal disease of varying etiologies, we immunostained human biopsies of immunoglobulin (Ig)A nephropathy, minimal-change nephrotic syndrome, focal segmental glomerulosclerosis, membranoproliferative glomerulonephritis, chronic pyelonephritis, and acute tubular necrosis (ATN) for SDF-1alpha, CXCR4, and CD45, a pan-hematopoietic marker. Irrespective of the diagnosis, intense SDF-1alpha immunoreactivity was localized to distal tubules and collecting ducts, whereas CXCR4 showed intense staining in both distal and proximal tubules. In addition, whereas varying degrees of CD45(+) cell infiltrates were observed in all biopsies, we found focal infiltrates of CXCR4(+) cells mostly localized to the corticomedullary junction only in ischemic ATN. This correlated with more extensive staining for SDF-1alpha in these sites. In all investigated renopathologic conditions, CD45+ leukocyte recruitment to the kidney seems not to be driven by SDF-1alpha/CXCR4 interaction. A contribution of SDF-1alpha for influx of CXCR4(+) cells in the vicinity of arcuate vessels is suggested only in human ATN.

Dosage-dependent requirement of BMP type II receptor for maintenance of vascular integrity

Germ-line mutations in bone morphogenic protein type II receptor (Bmpr2) confer susceptibility to pulmonary arterial hypertension (PAH), which is characterized by obstructive vascular lesions in small arteries. The molecular and cellular mechanisms that account for the etiology of this disorder remain elusive, as does the role of Bmpr2 in postnatal tissue homeostasis. Here we show that in adult mice, stably silencing Bmpr2 expression by RNA interference does not increase pulmonary arterial resistance but results in severe mucosal hemorrhage, incomplete mural cell coverage on vessel walls, and gastrointestinal hyperplasia. We present evidence that BMP receptor signaling regulates vascular remodeling during angiogenesis by maintaining the expression of endothelial guidance molecules that promote vessel patterning and maturation and by counteracting growth factor–induced AKT activation. Attenuation of this function may cause vascular dysmorphogenesis and predisposition to angioproliferative diseases. Our findings provide a mechanistic link between PAH and other diseases associated with the BMP/TGF-β pathways, such as hereditary hemorrhagic telangiectasia and juvenile polyposis syndrome.

The SDF-1-CXCR4 signaling pathway: a molecular hub modulating neo-angiogenesis

Pro-angiogenic bone marrow (BM) cells include subsets of hematopoietic cells that provide vascular support and endothelial progenitor cells (EPCs), which under certain permissive conditions could differentiate into functional vascular cells. Recent evidence demonstrates that the chemokine stromal-cell derived factor-1 (SDF-1, also known as CXCL12) has a major role in the recruitment and retention of CXCR4(+) BM cells to the neo-angiogenic niches supporting revascularization of ischemic tissue and tumor growth. However, the precise mechanism by which activation of CXCR4 modulates neo-angiogenesis is not clear. SDF-1 not only promotes revascularization by engaging with CXCR4 expressed on the vascular cells but also supports mobilization of pro-angiogenic CXCR4(+)VEGFR1(+) hematopoietic cells, thereby accelerating revascularization of ischemic organs. Here, we attempt to define the multiple functions of the SDF-1-CXCR4 signaling pathway in the regulation of neo-vascularization during acute ischemia and tumor growth. In particular, we introduce the concept that, by modulating plasma SDF-1 levels, the CXCR4 antagonist AMD3100 acutely promotes, while chronic AMD3100 treatment inhibits, mobilization of pro-angiogenic cells. We will also discuss strategies to modulate the mobilization of essential subsets of BM cells that participate in neo-angiogenesis, setting up the stage for enhancing revascularization or targeting tumor vessels by exploiting CXCR4 agonists and antagonists, respectively.

Expression of Vascular Endothelial Growth Factor, Stromal Cell-Derived Factor-1, and CXCR4 in Human Limb Muscle With Acute and Chronic Ischemia

OBJECTIVE

Vascular endothelial growth factor (VEGF)-induced stromal cell-derived factor-1 (SDF-1) has been implicated in angiogenesis in ischemic tissues by recruitment of CXCR4-positive bone marrow-derived circulating cells with paracrine functions in preclinical models. Here, evidence for this is provided in patients with peripheral artery disease.

METHODS AND RESULTS

Expression patterns of VEGF, SDF-1, and CXCR4 were studied in amputated limbs of 16 patients. VEGF-A was expressed in vascular structures and myofibers. SDF-1 was expressed in endothelial and subendothelial cells, whereas CXCR4 was expressed in proximity to capillaries. VEGF-A, SDF-1, and CXCR4 expressions were generally decreased in ischemic muscle as compared with nonischemic muscle in patients with chronic ischemia (0.41-fold, 0.97-fold, and 0.54-fold induction [medians], respectively), whereas substantially increased in 2 patients with acute-on-chronic ischemia (3.5- to 65.8-fold, 3.9- to 19.0-fold, and 4.1- to 30.6-fold induction, respectively). Furthermore, these gene expressions strongly correlated with capillary area. Only acute ischemic tissue displayed a high percentage of hypoxia-inducible factor-1alpha-positive nuclei.

CONCLUSIONS

These data suggest that VEGF and SDF-1 function as pro-angiogenic factors in patients with ischemic disease by perivascular retention of CXCR4-positive cells. Furthermore, these genes are downregulated in chronic ischemia as opposed to upregulated in more acute ischemia. The VEGF-SDF-1-CXCR4 pathway is a promising target to treat chronic ischemic disease.

Differentiation of arterial and venous endothelial cells and vascular morphogenesis

The vascular system is comprised of an organized hierarchical structure of arteries, veins, and capillaries. Recent studies in zebrafish, chick, and mouse reveal that the identity of artery and vein is governed by genetic factors as well as blood flow. The ephrin/Eph system establishes arterial and venous endothelial cell identity, and is important for structural segregation between arteries and veins. Analyses using loss- or gain-of-function mutations in zebrafish and mice show that Su(H)/RBP-J-dependent Delta/Notch signaling is a key mediator of arterial endothelial cell fate decision and vascular patterning. Vascular endothelial growth factor has also been shown to work upstream of Notch and is a key player in arteriogenesis. On the other hand, an orphan nuclear receptor, COUP-TFII, induces venous endothelial cell differentiation by suppressing the Notch signaling. Arteriovenous malformations are frequently induced by a loss of arterial and venous cell specification. These insights indicate that the balance of these genetic factors and modification by epigenetic factors such as hemodynamics and oxygen tension are important for proper endothelial cell identities in vascular morphogenesis.

Adrenomedullin/cyclic AMP pathway induces Notch activation and differentiation of arterial endothelial cells from vascular progenitors

OBJECTIVE

The acquisition of arterial or venous identity is highlighted in vascular development. Previously, we have reported an embryonic stem (ES) cell differentiation system that exhibits early vascular development using vascular endothelial growth factor (VEGF) receptor-2 (VEGFR2)-positive cells as common vascular progenitors. In this study, we constructively induced differentiation of arterial and venous endothelial cells (ECs) in vitro to elucidate molecular mechanisms of arterial-venous specification.

METHODS AND RESULTS

ECs were induced from VEGFR2+ progenitor cells with various conditions. VEGF was essential to induce ECs. Addition of 8bromo-cAMP or adrenomedullin (AM), an endogenous ligand-elevating cAMP, enhanced VEGF-induced EC differentiation. Whereas VEGF alone mainly induced venous ECs, 8bromo-cAMP (or AM) with VEGF supported substantial induction of arterial ECs. Stimulation of cAMP pathway induced Notch signal activation in ECs. The arterializing effect of VEGF and cAMP was abolished in recombination recognition sequence binding protein at the Jkappa site deficient ES cells lacking Notch signal activation or in ES cells treated with gamma-secretase inhibitor. Nevertheless, forced Notch activation by the constitutively active Notch1 alone did not induce arterial ECs.

CONCLUSIONS

Adrenomedullin/cAMP is a novel signaling pathway to activate Notch signaling in differentiating ECs. Coordinated signaling of VEGF, Notch, and cAMP is required to induce arterial ECs from vascular progenitors.